By 18 days after a missed period, the urine
pregnancy test does become reliable and conclusive. However, the test line showing fainter discolouration than control line is a fact of concern. This happens when during pregnancy, the levels of
hCG are low. This usually happens when the pregnancy is implanted somewhere outside the
uterine cavity - mostly tubes. This is called as ectopic pregnancy.
Ectopic pregnancy is a threatening condition. Usually it bursts if not intervened in time causing alarming bleeding. One has to be ectopic minded to suspect and diagnose ectopic pregnancy.
Your diagnosis of normal intrauterine pregnancy can be confirmed by
ultrasonography. If in the scan, the foetal sac is not shown in the uterine cavity but somewhere outside, you will require immediate surgery for it.
If the pregnancy is intrauterine, no need for any
stress.
I would advice you to report to your Obstetrician for clinical examination and further investigations and treatment which should include Ultrasonography.